The present invention relates broadly to internal combustion engines and, more particularly, to a mechanism for varying the stroke of an internal combustion engine over all four cycles, or "strokes", of engine operation. This mechanism acts to increase the efficiency of internal combustion engines by imposing a larger expansion stroke and a shorter intake stroke; meanwhile, the crankshaft arm will be gradually enlarged during the explosion or expansion stroke and gradually reduced during the admission or intake stroke. A modified thermodynamic cycle results from this particular piston movement.
The present invention acts to increase the efficiency of internal combustion engines. Fixed stroke internal combustion engines operate according to a predetermined cycle characterized by an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. In this sense, "stroke" is used to describe the movement of the piston through the maximum distance the piston travels during an excursion through the cylinder. It is also used herein to be the equal of "cycle", as when describing a four-cycle (or four stroke) engine, indicating that to complete a full power cycle the piston must reciprocate four times in the cylinder. "Cycle" is also used to describe the complete power cycle, such as the aforementioned Otto cycle. This usage is consistent within this art and, in context, should not confuse those so skilled.
A theoretical cycle of interest in analyzing the behavior of reciprocating spark ignition engines is the Otto cycle. FIG. 9 illustrates a pressure-volume (P-V) diagram of the Otto cycle superimposed with a P-V diagram of a modified cycle resulting from the present invention. The Atkinson cycle is characterized by complete expansion within the cylinder to exhaust pressure. When compared to conventional cycles, the Atkinson cycle offers a significant increase in efficiency, especially in engines with low compression ratios. The present invention offers the advantages provided by overexpansion due to its variable stroke feature.
Attempts have been made to make the piston travel a longer or shorter distance during the intake, compression, expansion, or exhaust strokes or during any combination of them. Also, the piston velocity has been modified in some portion of the piston travel, for every revolution or every two revolutions. The top or bottom dead center of the piston has been shifted up or down for every revolution or every two revolutions. All of these conditions are different versions of a variable stroke. Chadbourne U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 are patent documents that refer to an extended expansion stroke. A practical application of an extended expansion stroke is the Mazda Millenia, incorporating the Miller-cycle engine designed in 1947 by U.S. engineer Ralph Miller. Miller's engines have been used for some time in ships and stationary power plants. The engineering goal achieved is to reduce the engine's compression ratio without interfering with the power generating expansion stroke. In the Miller-cycle engine, the piston rises one-fifth of its stroke before the air intake valve is closed. After combustion occurs at the top of the stroke, the expanding gases push the piston all the way down to the bottom of the stroke, so the expansion ratio is not affected.
During the first half of the twentieth century, it was generally accepted that the combustion products inside an engine cylinder had to be removed as much as possible. Many different patents exist based in ways and means to obtain a larger exhaust stroke. Hulse U.S. Pat. No. 1,326,733; Svete U.S. Pat. No. 2,394,269; Cady U.S. Pat. No. 1,786,423; Tucker U.S. Pat. No. 1,964,096; and Austin U.S. Pat. No. 1,278,563 all emphasize this idea. Chadbourne U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 also refer to a larger exhaust and expansion stroke. However, due to new emission regulations, the idea of removing as much of the combustion products as possible is no longer the rule. Therefore, longer exhaust stroke is no longer stressed. On the contrary, during the last three decades, exhaust gas recirculation has been incorporated in most car engines. NOx (oxides of nitrogen) is a tailpipe emission caused by the oxidation of nitrogen in the combustion chamber. When the peak combustion temperatures get over 2500.degree. F., NOx is formed in excessive amounts. To keep the combustion temperatures down, exhaust gas is recirculated in most cars. This is accomplished by allowing intake manifold vacuum to draw exhaust gas into the intake manifold through an EGR (exhaust gas recirculation) valve. The present invention eliminates this device by allowing the adjustment of the top dead center of the piston (while ending the exhaust stroke) to retain the optimum amount of residual gas.
Others have used the variable stroke idea to modify the engine compression ratio. A lot of work has been done, especially in Europe and Japan, to achieve the so-called variable compression ratio. This is achieved by means of an arrangement that varies the position of the piston relative to the head of the cylinder.
The compression ratio is the ratio between capacity of the cylinder and capacity of the combustion chamber; in other words, the air-fuel mixture that goes into the cylinder during the admission stroke is then compressed as many times as the compression ratio value. Generally, the higher the compression ratio, the higher the engine efficiency. Some limitations such as mixture pre-ignition, knocking, engine temperature, and even engine construction exist. Since the compression ratio is one of the main factors affecting the engine efficiency, it is desirable to optimize it for different operation conditions (speed rate, load, acceleration, etc.). In some cases, this is performed automatically and in others in a selective manner according to the operating conditions. Schechter U.S. Pat. No. 5,165,368 is an example of the previous case. Note that the variation of the top dead center, when the piston is completing the compression or exhaust stroke is the same for a fixed operating condition. Therefore, the variation of the top dead center is a function of the operating condition and not a different thermodynamic cycle. In other words, the compression ratio and the expansion ratio have the same value. For the present invention, the compression ratio is different from the expansion ratio.
Another example of variable stroke application is the optimization of the pressure acting on the piston. To achieve this, the piston speed is decreased, relative to the speed of a conventional piston, near the top dead center to maximize the combustion process and the resulting forces acting on the piston. Schaal et al U.S. Pat. No. 5,158,047, Williams U.S. Pat. No. 5,060,603, and McWhorter U.S. Pat. Nos. 3,686,972; 3,861,239; and 4,152,955 are related to this variable stroke group.